Collagen, which constitutes about 40% of the protein in a living being, is the support substance for conjunctive tissue, and is necessary for the tissue to function.
Collagen possesses remarkable mechanical properties, has good hemostatic properties, and exerts an influence on cellular growth.
In addition to these properties, there are two other interesting collagen characteristics: its biocompatibility and its biodegradability.
The collagen macromolecule having a length of about 3000 .ANG. and a width of about 15 .ANG. is constituted by three peptide chains. Each chain has a mass of 100,000 daltons, and is in helicoidal form. The axes of the helices extend helically around a common axis through the interior of the macromolecule. Between certain peptide chains, there exist reticulated or cross-link bonds. The ordered arrangement of the macromolecules between these peptide chains leads to formation of fibers.
The excellent mechanical properties of collagen are provided in large part, by the helicoidal structure and the reticulated bonds.
The antigenic character of collagen is also very low. Consequently collagen originally from an animal does not provoke an action of rejection when applied in vivo to a human being, according to a study conducted by Takeda, U. et a and appearing in the Journal of Toxicology Sciences, Vol. 7, Suppl. II, pp 63-91 (1982).
There are two important advantages to collagen which make it readily adaptable to be used in vascular prostheses: on the one hand its biocompatibility and on the other hand its ability to exert an action in cellular growth. Conversely, in these applications, there are two drawbacks: its biodegradability and its hemostatic ability. This latter property is especially troublesome because of the risk of provoking thromboses and there is also the problem of the disappearance of the prosthesis by destruction of the helicoidal structure of the protein which results in poor mechanical properties. Finally the biodegradability of the collagen must be sufficiently low so that it can be reestablished by the cells before it is digested by the enzymes.
It is known that synthetic vascular prostheses are not really satisfactory if their diameters are greater than 4 mm. Above this dimension, the replacement vessel possess a serious inconvenience, in particular their level of effectiveness is low. That is why it is obviously important to find a way to fill this need with the aid of a collagen-based biomaterial which will be of some help in the field of vascular prostheses.
Previously the use of resinous heterografts has been revealed to be an asset and has brought about some interesting results.
Unfortunately, this technique is very costly and the number of applications to the veins is very limited. That is why it is necessary to be able to prepare tubes of a small diameter based on collagen.
Romanian Patent No. 76 922 describes a process and an apparatus to obtain tubular elements starting from collagen gel; in this process the collagen is put into solution and subjected to dialysis using distilled water to which is added EDTA and a tubular element containing collagen is thus formed by an electrode position technique. Besides the fact that such a technique is difficult to put into practice, it may be noted that the Romanian inventors do not give any indication of what is the minimum diameter of the tubes which are obtained by such a process.
In a lecture presented by Chigner, E., Huc, A. and Eloy, R. at the Eighteenth Congress of the European Society of Surgical and Stress Research in May 1982, the biocompatibility of collagen was emphasized and the ability of this material to function in making vascular prostheses was determined according to the following tests:
A sample constituted by a collagen very close to that of the collagen in the tubes described hereinafter was sutured onto the aorta of a rat, the aorta having previously been perforated. The results obtained indicated a good biocompatibility of the material, the absence of blood coagulation upon contact with the collagen, and finally a mechanical resistance sufficient to withstand blood pressure.